One kind of apparatus for measuring the amount of fuel in a tank comprises a capacitive probe disposed in the tank to have a capacitance that is a function of the amount of fuel in the tank; an electronic circuit connected to the probe to provide an output signal that is a function of the capacitance of the probe; and display means responsive to the output signal to provide an indication of the amount of fuel in the tank.
In apparatus of the above defined kind, the probe may comprise a pair of spaced coaxial electrodes constituting a cylindrical plate capacitor between the plates of which the fuel enters to vary the probe capacitance as a function of fuel level, assuming the capacitor is mounted to have its axis vertical. The probe thus acts as a capacitive transducer. The capacitance is at a maximum value when the tank is full, and at a minimum value when the tank is empty.
The tank may be of irregular or non-uniform cross-section such that the amount of fuel in the tank is not linearly proportional to the fuel level. The probe may be given a capacitance profile, i.e. capacitance as a function of fuel level, that is adjusted to the shape of the tank. When the capacitance varies essentially in accordance with the amount of fuel, this will be termed a "matched profile". The matched profile can be achieved by having a smooth cylindrical outer electrode or plate and an inner electrode or plate of varying diameter along the axis so as to vary the radial distance between the plates. Where the electrodes are both smooth cylindrical surfaces, i.e. a uniform profile, this will match a tank of uniform cross-section.
In order to sense the changes in probe capacitance and accordingly the amount of fuel, the probe plates are connected in an electronic measuring circuit which in most applications comprises a bridge circuit located remotely from the probe, for instance in the vehicle cabin. The bridge or similar circuit, which supplies an analog signal, is then connected to a display device, for example a milliammeter, arranged on the vehicle dashboard or instrument panel, so that the vehicle driver or pilot can at any instant ascertain the amount of fuel in the tank.
These conventional systems suffer from the disadvantage that the analog signal to be measured is small and readily subject to disturbance due to the fact that the capacitance to be measured is of a relatively small value and the probe capacitor is remote from the measuring circuit.
The probe capacitance is typically in the range of 50-100 pF, as it is impossible for overall size and weight reasons to increase the plate dimensions, nor is it possible for reasons of safety to reduce the capacitor plate spacing below a certain limit.
On the other hand, the distance separating the probe from the measuring circuit may be as much as 15-20 meters. For example in an aircraft the tanks having the probes immersed therein are located on the wings or in the fuselage of the aircraft while the measuring circuit is normally placed in the cabin. Under such circumstances the capacitance measure may be inaccurate.
The measurement of the analog signal is disturbed by signals induced on the cable or wire connecting the probe to the measuring circuit, and which are caused by external electromagnetic fields, such as generated by the vehicle radio set. The measurement is also affected by the possible variations in capacitance of the connecting cable or wire. In order to reduce these disturbances, the use of coaxial cables is often resorted to, which adds to the total cost of the installation.
On the other hand, in the conventional systems it is not possible to ground one probe capacitor plate in order to provide protection against disturbances, as the measuring circuit does not allow grounding of the probe plates.
A further disadvantage of the prior systems is that a test of correct operation of all components of the system under the effective operating conditions cannot be carried out. Thus, in the prior art systems, the actual capacitance of the probe is replaced during this test by a fixed capacitor and, as a result, the probe capacitor is not connected in the system, so that the testing of the circuit operation does not check the probe as well.
Particular importance is placed on measurement accuracy where aircraft are connected where serious consequences arise if erroneous signalling indicates to the pilot a fuel reserve that does not exist in reality.
It will be shown hereinafter how the present invention can be put into practice for sensing and signalling the amount of fuel in a tank, particularly in an aircraft tank, so as to allow accurate measurement of the capacitance of the probe, even in the presence of strong disturbing electromagnetic fields and with a probe located at a considerable distance from the display device, whereby the pilot can be given an accurate indication of the fuel in reserve. The apparatus to be described has good protection against radio disturbances or noises, so as to be relatively insensible to interference.
The preferred apparatus of the invention may be implemented with critical components mounted in a unitary structure located directly on the fuel tank and designed in such a way as to allow a test of correct operation of the system, while all of the components, probe included, are energized.
In addition it will be shown how the invention may be put into practice either with a probe having a capacitance profile matching that of the tank in which is is located or with an unmatched probe, e.g. a smooth profile probe in a non-uniform tank, by taking other measures. Still further the apparatus to be described can be constructed in a way capable of supplying suitable information for processing by a vehicle computer.